L-Glutamate, which is the most abundant neurotransmitter in the CNS, mediates the major excitatory pathway in mammals, and thus is referred to as an excitatory amino acid (EAA). The receptors that respond to glutamate are called EAA receptors. The EAA receptors are of great physiological importance playing a role in a variety of physiological processes, such as long-term potentiation (learning and memory), the development of synaptic plasticity, motor control, respiration, cardiovascular regulation, emotional states and sensory perception.
The excessive or inappropriate stimulation of EAA receptors leads to neuronal cell damage or loss by way of a mechanism known as excitotoxicity. The medical consequence of such neuronal degeneration makes the abatement of these degenerative neurological processes an important therapeutical goal.
EAA receptors are classified into two general types. Receptors that are directly coupled to the opening of cation channels in the cell membranes of the neuron are termed "ionotropic". The second type of receptor is the G-protein or second messenger-linked "metabotropic" EAA receptor. This second type is coupled to multiple second messenger systems that lead to enhanced phosphoinositide hydrolysis, activation of phospholipase D, increases or decreases in cAMP formation, and changes in ion channel function. The metabotropic glutamate receptors (mGluR) have been distinguished pharmacologically from the ionotropic glutamate receptors by the use of the metabotropic glutamate-selective antagonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, generally through measurements involving phosphoinositide hydrolysis or Ca.sup.2+ mobilization. To date the use of expression cloning techniques has led to the identification of eight mGluR subtypes, which have been placed into three major categories (Groups) based on their molecular structure, signal transduction mechanisms, and pharmacological properties. Group I mGluRs (mGluR1 and 5) are coupled to phosphoinositide (PI) hydrolysis, whereas Group II (mGluR2 and 3) and Group III (mGluR4, 6, 7, and 8) are negatively linked to adenylyl cyclase activity.
Glutamate receptors play a role in numerous neurological, neurodegenerative, psychiatric, and psychological disorders, and a variety of mammalian disease states are associated with "abnormal activity" of these receptors. As used herein, the term "abnormal activaty" includes either an increase or a decrease in activation of the receptors as compared to normal function in said mammal. For example, metabotropic glutamate receptor ligands should be useful in the treatment of diseases or conditions including epilepsy, cerebral ischemia, pain, anxiety, spinal cord injury, chronic neurodegenerative diseases (e.g. Alzheimer's disease), Lou Gherig's disease (ALS), Parkinson's disease, Multiple Sclerosis and other diseases or conditions that result in progressive loss of neuronal cells and or cellular function.
In order to better characterize the roles of mGluRs in physiological processes, there is a need to identify novel, high affinity compounds that are mGluR Group or subtype specific. Such compounds are needed for use as pharmacological tools for the further investigation of mGluR function, and should be useful as therapeutic agents for the treatment of diseases or conditions associated with abnormal activity of metabotropic glutamate receptors.